Method and arrangement for assessing the quality of skin print images

ABSTRACT

In a method and an arrangement for assessing the quality of skin print images, and particularly fingerprint images, provision is made for gradients to be formed for the individual picture elements (pixels) of the skin print image, for a mean value to be formed from the gradients of the pixels in one region of the image (tile) at a time, and for similarities in the mean values from tile to tile to form a measure of quality.

The invention relates to a method of assessing the quality of skin printimages, and particularly fingerprint images. The invention furtherrelates to an arrangement for carrying out a method of this kind.

When images are made of fingerprints or other skin prints, there may bevarious factors that have an adverse effect on their quality and, bothwhen a set of reference prints is being assembled and when fingerprintsare subsequently checked or identified, these may cause problems thateither make any checking or identification impossible or may even leadto erroneous results. Depending on the principle and construction of thesensors used, the quality of the images made may be adversely affectedby dirt or moisture, grease on the finger, or a finger that is too dry.Dirt and fouling in or on the sensor itself is another possible cause ofimages of poor quality of prints from the skin. Added to this, there isalso the possibility of incorrect operation by the user himself.

Inferior quality images of skin prints generally result in the imagesbeing rejected but, if the worst comes to the worst, they may result inthe system being compromised if, due to its poor quality, the skin printis wrongly accepted as correct. It is therefore important to be able toassess the quality of the image that is made of a fingerprint.

It is therefore an object of the present invention to specify a methodof determining the quality of images of skin prints, and particularly offingerprint images. It is also an object of the invention to provide anarrangement for carrying out a method of this kind.

This object is achieved, in accordance with the invention, in thatgradients are formed for the individual picture elements (pixels) of theskin print images, in that a mean value is formed from the gradients ofthe pixels in one region of the image (tile) at a time, and in thatsimilarities in the mean values from tile to tile form a measure ofquality.

In calculating gradients, which in the end are to specify the directionsof the ridges, also gradients in the opposite direction are calculated,though these too in the end represent an identical direction of theridges. To eliminate the sign when this is done, provision is made in anadvantageous embodiment of the invention for the gradients formedinitially, which have the components g_(x(alt)) and g_(y(alt)), to besquared after the fashion of complex numbers by the formulasg_(x)=g_(x(alt)) ²−g_(y(alt)) ² and g_(y)=2g_(x(alt))*g_(y(alt)).

It is assumed for the purposes of the invention that in skin printimages the direction of the ridges changes only slightly from one tileto the adjacent one. This presupposes that the complete skin print imageis divided into a sufficiently large number of tiles, which is necessaryanyway for a later analysis, and in particular for an extraction of thecharacteristic features. In this way, the size of the tiles may be 10×10pixels, for example.

A particularly advantageous manner of determining whether the meanvalues change to a greater or lesser degree from tile to tile comprises,in another embodiment, entering the mean values in two directionalmatrices for x and y, in that scalar products are formed of thedirectional matrices together with the matrices that are displacedhorizontally, vertically and in the directions of both diagonals by onetile, in that each of the products that were obtained in that way bymultiplying the matrices are summed over all the tiles, and in that thesums are added together and are divided by the sum of the scalarproducts of the directional matrices with themselves in order to formthe quality measure, said sum of the scalar products of the directionalmatrices with themselves being summed up over all tiles.

The sensor usually covers a rectangular area, in which case the fingerdoes not rest on the sensor in the edge or corner regions. The areasthat are thus not occupied by the finger may in certain cases prove anuisance when quality is being assessed. This is not so much the casewith sensors in which the areas not occupied by the finger, i.e. theareas lying outside what is termed the region of interest (ROI), arehomogeneous. However, use is also made of sensors that produce a rasterpattern or stripes outside the region of interest. With sensors of thistype, it is advisable for the assessment of quality to be confined tothe region of interest.

To enable the region of interest to be determined for this purpose,provision may be made, in another embodiment, for the lengths of thegradients for which mean values have been formed to be used to determinea region of interest of the skin print that has been scanned.

The arrangement according to the invention is advantageously arranged tohave digital signal processing means to perform the method stepsaccording to the invention. Recourse may be had in this case to systemsthat are known per se for individual steps of the signal processing.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 is a diagrammatic representation of the gradients determinedwithin a tile.

FIG. 2 shows mean values of the gradients determined over respectivetiles, for the entire image, and

FIG. 3 shows equations for calculating the quality measure.

For the sake of clarity, the tile 1 shown in FIG. 1 has only 4×4 pixels.Within the tile there are—once again as a major simplification—parts oftwo ridges 2, 3 that, for the purposes of the rules relating to drawingsfor patents, have been shown as hatched bars. Also, no allowance hasbeen made in the representation either for the characteristic followedby the gray levels or for any local quantization by the scanningprocess. For each pixel, there is calculated by methods that are knownper se a gradient 4 that—as described above—does not depend on the signof the edges of the ridges 2, 3. Since the ridges do not have aparticularly pronounced curvature at the majority of points in the skinprint image, the gradients assume substantially similar directions.

The length of the gradients too is not a faithful representation ofreality. It has merely been indicated that the gradients may be ofdifferent lengths. For further processing, the components in the x and ydirections are placed in respective directional matrices.

As an illustration, FIG. 2 shows the entire image of a skin printcontaining representative mean values of the gradients calculated forrespective ones of the tiles 1.

As already mentioned, the region of interest 5 does not cover the wholeof the area scanned by the sensor. If the sensor used is one that,outside the region of interest 5, produces a homogeneous signal, such asblack or white for example, then it is perfectly possible for the entirearea 6 to be covered in the subsequent steps of the method according tothe invention. In the tiles that are situated outside the region ofinterest, the mean values obtained for the gradients then become zeroand hence are no nuisance. The same is true for tiles of which only partis situated in the region of interest 5.

To form the quality measure, an autocorrelation is performed of the meanvalues g. For this purpose, scalar products A_(x), A_(y), A_(xy) andA_(yx) are formed using the equations shown in FIG. 3. Also, the value Ais derived as a center for the autocorrelation. The quality measure Q isthen calculated as Q=(A_(x)+A_(y)+A_(xy)+A_(yx))/4A.

The summing is performed in each case for the entire area 6, namely fromk=1 and 1=1 to their maximum values.

1. A method of assessing the quality of skin print images, andparticularly fingerprint images, characterized in that gradients areformed for the individual picture elements (pixels) of the skin printimages, in that a mean value is formed from the gradients of the pixelsin one region of the image (tile) at a time, and in that similarities inthe mean values from tile to tile form a measure of quality.
 2. A methodas claimed in claim 1, characterized in that the gradients formedinitially, which have the components g_(x(alt)) and g_(y(alt)), aresquared after the fashion of a complex number by the formulasg_(x)=g_(x(alt)) ²−g_(y(alt)) ² and g_(y)=2g_(x(alt))*g_(y(alt)).
 3. Amethod as claimed in claim 1, characterized in that the mean values areentered in two directional matrices for x and y, in that scalar productsare formed of the directional matrices together with the matrices thatare displaced horizontally, vertically and in the directions of bothdiagonals by one tile, in that each of the products that were obtainedin that way by multiplying the matrices are summed over all the tiles,and in that the sums are added together and are divided by the sum ofthe scalar products of the directional matrices with themselves in orderto form the quality measure, said sum of the scalar products of thedirectional matrices with themselves being summed up over all tiles. 4.A method as claimed in claim 1, characterized in that the lengths of theaverage gradients are used to determine a region of interest of the skinprint that has been scanned.
 5. An arrangement for assessing the qualityof skin print images, and particularly fingerprint images, characterizedby a system for forming gradients for the individual picture elements(pixels) of the skin print image, a mean value from the gradients of thepixels in one region of the image (tile) at a time, and a measure ofquality from similarities in the mean values from tile to tile.
 6. Anarrangement as claimed in claim 5, characterized in that the system forforming the gradients for the individual picture elements (pixels) ofthe skin print image and for forming the mean value from the gradientsof the pixels in one region of the image (tile) at a time is arranged tosquare initially formed gradients, which have the components g_(x(alt))and g_(y(alt)), after the fashion of a complex number by the formulasg_(x)=g_(x(alt)) ²−g_(y(alt)) ² and g_(y)=2g_(x(alt))*g_(y(alt)).
 7. Anarrangement as claimed in claim 5, characterized in that the system forforming the measure of quality from similarities in the mean values fromtile to tile is arranged to enter the mean values in two directionalmatrices for x and y, to form scalar products of the directionalmatrices having matrices that are displaced horizontally, vertically andin the directions of both diagonals by one tile, to sum, over all thetiles, each of the products that are obtained by multiplying thematrices, and to form the quality measure by adding the sums togetherand dividing the scalar products of the directional matrices, saidscalar products having been summed over all the tiles, by themselves. 8.An arrangement as claimed in claim 5, characterized in that the systemfor forming gradients for the individual picture elements (pixels) ofthe skin print image and for forming a mean value from the gradients ofthe pixels in one region of the image (tile) at a time is arranged todetermine, from the lengths of the averages gradients, a region ofinterest of the skin print that has been scanned.